[ADT/MathExtras] Introduce PowerOf2Ceil.

To be used in lld (and probably somewhere else in llvm).

Differential Revision:  https://reviews.llvm.org/D26538

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286549 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed